It is sometimes desirable to have the capability of synchronizing together two or more video systems. For example, in a television news broadcasting studio, it may be desirable to switch between a "live" presentation of the news commentators on one camera, a videotape on a second camera, and a slide presentation on a third camera. In order to accomplish this switching without diminishing pictures quality, it is necessary to synchronize the operation of all three cameras.
More particularly, if independent video systems do not operate in continuous synchronization, switching between one video system and the other for broadcast transmission can cause the video picture to be adversely affected, such as by rolling in the vertical and tearing in the horizontal. This can occur until the systems have become synchronized together.
Conventionally, master sync generator have been incorporated into video cameras. A conventional method of syncing two or more cameras is to have a lead camera set to an internal mode for generating a composite video signal, with the sync generators in the remaining cameras locked to this video signal.
In an exemplary embodiment of the present invention, there is provided a video system which utilizes an image sensor such as a charge coupled device (CCD) to generate video images. It is an object of the present invention to synchronize the imager to an external video signal. However, the CCD sensor utilizes a clocking signal which has a much higher frequency than is present in a conventional video signal. Therefore, in order to minimize the differential between the imager clocking frequency and the external video frequency, the highest frequency portion, i.e., color burst, of the external video signal is utilized whenever possible as a reference to generate the signal for clocking the imager. However, in the absence of a color burst signal in the external video such as during the transmission of black and white video, or during the vertical blanking interval, it is desirable that the next highest frequency available in the composite video signal, i.e. horizontal sync information, be utilized as a reference for generating the CCD clocking signal.
A number of conventional synchronization systems have been disclosed. For example, in U.S. Pat. No. 4,555,679 by Katsuyama there is provided a system which changes the freqency band characteristics of a phase lock loop depending upon whether the PLL is in a synchronous or an asynchronous state.
Okey, in U.S. Pat. No. 3,969,758 discusses a synchronizing circuit which utilizes two oscillators, one of which is phase locked to the color burst of a composite video signal, and the other of which is phase locked to the horizontal sync information of the composite video signal.
A playback apparatus for reproducing a composite color video signal is disclosed in U.S. Pat. No. 4,l45,705 by Yoshinaka in which there is provided a phase lock loop circuit for regenerating a carrier reference signal in response to horizontal synchronizing signals of a composite color video signal.
In U.S. Pat. No. 4,122,488 by Mikado there is provided a gen-lock system which generates a stabilized synchronizing signal even when an external reference signal includes noise or is temporarily interrupted.
A digital video processing system disclosed in U.S. Pat. No. 4,612,568 by den Hollander et al., converts an analog video signal to a digital format utilizing sampling clock signals which are phase locked to the color burst information of a video signal.
Arimura et al., in U.S. Pat. No. 3,733,432 discloses a system for producing a continuous signal in synchronous phase with the color burst signal of a composite video input by means of an oscillator whose oscillation frequency is approximately equal to that of the burst signal.
In U.S. Pat. No. 4,675,724 by Wagner, there is disclosed an analog phase lock loop which operates at four times the subcarrier frequency of an input video signal and which feeds back a four-times subcarrier sampling clock to an adjustable analog-to-digital converter for time-based correction of the signal.
D'Hont in U.S. Pat. No. 4,639,765 provides a synchronization system which includes an internally generated video signal which is synchronized to an external video signal.
A system which has a first clock output generated by a phase lock loop synced to a horizontal sync signal, and a second clock output synchronized to a color burst signal, is disclosed by Takamori et al. in U.S. Pat. No. 4,613,827.
Furthermore, in U.S. Pat. No. 4,468,687 by Munezawa et al., there is provided a synchronizing system which separates synchronizing signals from a composite video signal and utilizes the separated synchronizing signals to remove color burst information from a video input.
And further more, in U.S. Pat. No. 4,038,683 by Thorpe et al., there is disclosed a system for synchronizing the video signal output of a television camera to an external referency utilizing first and second phase lock loops for maintaining synchronism of the line and color signal components of the video signal with the referency.